If the worst possible thing happens, stay calm.
Despite all the machismo talk on ball-shaving forums about how dudes use chainsaws to trim their pubes and don't give a fuck about cuts, nicking your D is no laughing matter. First, stop screaming and get your hands on a band-aid or a cotton ball or something. Apply gentle pressure to stop the bleeding, and apply some Neosporin or triple antibiotic ointment to keep the area clean. The last thing you want is an infection fucking your shit up, so keep one eye on your one-eyed monster, and if anything develops that reminds you of last spring break's fallout, cancel everything in your life and go see a doctor.